Insights on dissolved organic matter and bacterial community succession during secondary composting in residue after black soldier fly larvae (Hermetia illucens L.) bioconversion for food waste treatment

Black soldier fly larvae (Hermetia illucens L. BSFL) bioconversion is a promising biotechnology for food waste treatment. However, the separated residues still do not meet criteria for use as land application biofertilizers. In this work, we investigated a full-scale BSFL bioconversion project to explore features of dissolved organic matter (DOM) and its associated responses of bacterial community succession in residue during secondary composting. Data showed that the concentrations of total nitrogen and ammonium nitrogen decreased by 11.8% and 22.6% during the secondary composting, respectively, while the nitrate nitrogen concentration increased 18.7 times. The DOM concentration decreased by 69.1%, in which protein-like, alcohol-phenol, and biodegradable aliphatic substances were metabolized by bacteria during the thermophilic phase together with the accumulation of humus-like substances, resulting in an increase in the relative concentration of aromatic compounds. The structure of the bacterial community varied at different stages of the bioprocess, in which Bacteroidetes, Actinobacteria, Proteobacteria, and Firmicutes were the dominant bacterial phyla. Lysinibacillus, Pusillimonas, and Caldicoprobacter were found to be key contributors in the degradation and formation of DOM. The DOM concentration (33.4%) and temperature (17.7%) were the prime environmental factors that promoted succession of the bacterial community. Through bacterial metabolism, the structural stability of DOM components was improved during the composting process, and the degrees of humification and aromaticity were also increased. This study depicted the dynamic features of DOM and the associated bacterial community succession in residue during secondary composting, which is conducive with the reuse of BSFL residue as biofertilizer for agriculture.